The ordinate of the endpoint of an arc of a unit circle centered at the origin of a Cartesian coordinate system, the arc being of length x and measured counterclockwise from the point (1, 0) if x is positive or clockwise if x is negative.
In a right triangle, the ratio of the length of the side opposite an acute angle to the length of the hypotenuse.
[Medieval Latin sinus (mistranslation of Arabic jayb, sine, as if jayb, fold in a garment), from Latin, curve, fold.]
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